2018-10-07

Steven WeinbergのTo Explain The Worldを読んだ

邦訳では「科学の発見」という題名がついているSteven WeinbergのTo Explain The Worldを読んだ。

著者のSteven Weinbergはノーベル物理学賞の受賞者でイスラエルの熱烈な支持者だ。

冒頭で「本書で筆者は過去を現代の基準で批判する愚を犯す」と書きながら、この本は科学がどのように発展してきたかを解説している。

本書はまず、古代ギリシャにおける万物を構成する元素の説について取り上げる。古代ギリシャの哲学者が、元素は水だとしたり、火だとしたり、水、火、土、空気の4種類だと主張している歴史を取り上げる。

ここまではまあいいとして、プラトンの提唱した説を取り上げて悶絶する。プラトンは四大元素である火、水、土、空気について、とても小さい元素が存在し、5種類ある正多面体をそれぞれ割り当てた。たとえば火は正四面体で、水は正二十面体といった具合だ。その仮説を立てるのはいいとして、割り当ては一体どういう理由で行われたのか。プラトンは単にそれが理想だからとしか語らない。

科学というものは、まず観測し、観測結果から仮説を立て、仮説が観測結果に従うことを検証するものだ。

しかし古代ギリシャでは、科学は科学ではなく哲学で、単に理想を追求するものでしかなかった。なので万物を構成する元素は元素は火や水といったわかりやすい理想的なものになり、元素は5種類ある正多面体であるとし、完全数を尊びといった、現実の観測結果より理想を追求し、理想が現実に従わないことは無視されていた。

その後、本書の大半は天体の運行の予測を通じた科学の発展に費やされる。

天体の運行には規則性があり、古代から占星学などの存在により、天体の運行を正確に予測する需要があった。

古代ギリシャ時代から地球が中心で月、惑星、太陽、その他の恒星は全て地球の周りを回っている説が主流であった。

古代ギリシャ時代にはすでに地球を中心とした天体の運行を予測する数式モデルが考案されていたが、他の天体はすべて地球を中心とした真円で回るなどと定義されていたため、現実の観測とは大いに異なっていた。しかも、それぞれの天体が謎の理由で割り当てられた正多角形に内接する真円で回るなどという、これまたプラトンのように理想を追求したモデルであった。

天体は地球を中心に真円で回転するモデルを使うと、惑星はある時期だけ逆方向に回りだしたりする。惑星が大抵の言語で語源からして、惑う星であるのも、これが原因だ。

そこでプトレマイオス説が考案された。この説では、天動説で現実の観測結果に近似させるために、あまりにも無理やりな天体のモデルを考案した。天体は太陽を中心に真円で回っている。この真円を従円と呼ぶ。天体は従円を線上を中心としてさらに真円で回っている。この真円を周転円と呼ぶ。

プトレマイオス説は複数の真円を組み合わせ、真円の大きさをパラメーター化することで、惑星の運行を現実に近似させることを試みた。その結果、かなりの制度で現実の観測に近似した天体モデルを作ることに成功した。

プトレマイオス説は科学だ。観測から観測結果に従う数式モデルを作ったわけで、これは科学と言える。

プトレマイオス説は哲学者からは理想ではなく天体の予測のための方便であるとされていた。天文学者(この時代の天文学者は占星学をかねる)は積極的に使っていた。というのも、プトレマイオス説は現実の観測結果の近似していて、予測に役立つからだ。

その後、様々な天文学者がプトレマイオス説により複雑な周転円を追加することで、さらに現実の観測結果に近づける努力が行われた。

太陽を中心として天体が公転している地動説は以前にも理想として提唱されたことはあったが、現実の観測結果を説明できる具体的な数式モデルはコペルニクスによってまず作られた。ただし、コペルニクスの数式モデルは精度が悪かった。というのも、コペルニクスは天体は太陽を中心に真円で公転していると定義したが、実際には太陽は中心ではなく、天体の公転軌道は真円ではないからだ。

本書は中東の科学についても解説している。中東の科学はイスラム教の普及によって妨害されるまでヨーロッパより進んでいた。ギリシャ時代の学説は中東からもたらされる形でヨーロッパに再発見された。その後イスラム教の普及により中東の科学の発展は阻害された。

そしてガリレオ・ガリレイの時代までやってくる。ガリレオ・ガリレイは優秀な望遠鏡を発明し、望遠鏡によって精密な天体観測をした。その結果地動説を唱えるわけだが、その上でパトロンであるローマ教皇を天動説を信じているなどと批判したので宗教裁判にかけられて地動説を封印する。後世に残る逸話によれば、このときガリレオは「それでも地球は回っている」とつぶやきながら法廷を後にしたと言われる。

宗教裁判の後、ガリレオは晩年まで自宅に軟禁状態におかれるわけだが、ガリレオの科学への情熱は冷めていなかった。ガリレオは落下する物体の速度について研究していた。落下する物体の速度を観測するのは当時としては難しい。というのも、速度が速すぎるために正確な観測ができないためだ。

ガリレオはこの落下速度の問題に対し、科学的な観測方法を考案する。緩やかな傾斜面を転がる球を使うことで速度の計測を可能にした。時間の計測には水時計を使った。傾斜角を変えることでガリレオは速度は傾斜角に比例することを示し、傾斜角を転がる球は落下速度の計測の代わりに使うことができることを示した。もちろん現実には、球と傾斜面の摩擦にもエネルギーが使われるが微々たるもので当時実現可能な観測方法としては十分なものだった。ガリレオは傾斜角を転がる球を机の端から飛ばし、その軌道が放物線であることも観測している。

本書はニュートンの説明に移る。ニュートンによって科学革命はクライマックスを迎えるわけだが、しかしこのニュートンという人物はなんという奇人だろうか。ニュートンは生涯、イングランドのごく狭い地域より外に出ることはなかった。ニュートンは潮の満ち引きについて多大なる関心を持っていたにもかかわらず、生涯一度も海を見ることはなかった。中年に至るまでニュートンは身近に女を寄せ付けることがなかった。母親とて例外ではない。50代になってから親戚の美しい娘を家政婦として雇っているが、この2人の間に男女関係はなかった。ニュートンは科学以外にも、非科学的な錬金術や宗教について多大な著作を残している。

ニュートンの研究者の間でよく言われることには、ニュートンは最初の科学者ではない。ニュートンは最後の魔法使いである。時代が魔法から科学に変わる節目の時期にあって、魔法から科学への橋渡しをした人物だ。

ニュートンの法則を記述したPrincipiaは、単に重力を説明し得たために偉大だというわけではない。ニュートンはPrincipiaによって、物理的現象は簡単な数学的原則と、その原則の応用で説明できることを示したのだ。

70 comments:

Anonymous said...

制度→精度

said...

この時代、旅は大変で命がけでした。バッハも
ニュートンみたいだったようですが、こちらは20人も
子供がいたそうです。ニュートンというと微分積分を連想して
しまいますが、ほんとにすごい人ですね。

said...

そうそう、最近物理がらみで、もしやって
思ったことがあります。量子ゆらぎってありますよね
これがあると、原子の中の重力のバランスというか
モーメントも揺らぐじゃないですか。で、宇宙全体として
バランスを取ろうとして、重力をやり取りするのでは
なーんて、思ったのです。まあ、冗談ですけどね

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